1) Field of the Invention
The invention relates to a plasma arc torch and method for switching between a working mode and a standby mode and, more specifically, a standby mode characterized by an arc extending between an electrode and a nozzle, a reduced arc current, a standby gas, and/or a reduced gas flow rate.
2) Description of Related Art
Plasma arc devices are commonly used for cutting and welding. One conventional plasma arc torch includes an electrode positioned within a nozzle. A pressurized gas is supplied to the torch and flows between the electrode and the nozzle, and an arc is established between the electrode and a workpiece. The arc ionizes the gas, and the resulting high temperature gas can be used for cutting or welding operations.
Erosion reduces the useful life of the electrode and is known to occur during transfer or operation of the torch (operation erosion) and during starting and stopping of the arc (start erosion). One typical method for starting the torch is to first initiate a pilot mode by establishing an arc at a low current between the electrode and the nozzle. The torch is then switched from the pilot mode to a transfer or working mode by transferring the arc to the workpiece so that the arc extends between the electrode and the workpiece, and increasing the current of the arc. A non-oxidizing gas can be supplied to the torch during the pilot mode to reduce the oxidation and erosion of the electrode, and an oxidizing gas can be supplied thereafter during operation. The use of a pilot mode is further described in U.S. Pat. No. 5,017,752, titled xe2x80x9cPlasma arc torch starting process having separated generated flows of non-oxidizing and oxidizing gas,xe2x80x9d assigned to the assignee of the present invention and the entirety of which is incorporated herein by reference.
Although the erosion of the electrode can be reduced by supplying the non-oxidizing gas to the torch during the pilot mode, the starting and stopping of the torch are still erosive to the electrode. Start erosion can constitute a significant source of total erosion of the electrode, for example, when a cutting torch is turned on and off repeatedly to cut a number of different workpieces or to make a number of discontinuous cuts in a single workpiece. One proposed method of reducing the start erosion attributable to such repeated starts is to maintain the arc between successive cuts instead of stopping and restarting the arc between each cut. The arc can be maintained by switching the arc from the workpiece to the nozzle or a special electrode so that the arc extends between the electrode and the nozzle or the special electrode. The start erosion of the electrode can be reduced using such a continuous arc, but the arc causes erosion of the nozzle or special electrode, especially if maintained for lengthy durations. Additionally, the provision of the special electrode on the torch increases the cost and complexity of the torch.
Thus, there is a need for an improved apparatus and method for reducing the erosive effects of the arc on both the electrode and nozzle. The apparatus should be capable of performing a number of discontinuous welding or cutting operations and maintaining a continuous arc between successive operations. Preferably, the apparatus should not require a special electrode for maintaining a continuous arc between cutting or welding operations.
The present invention provides a plasma arc torch and an associated method for switching between a working mode and a standby mode, which can be employed between successive welding or cutting operations. In the standby mode, the arc is switched to extend between the electrode and the nozzle. Additionally, the arc current is reduced and at least one flow parameter of the plasma gas is adjusted, for example, by changing the plasma gas composition and/or reducing the gas flow rate. Thus, the arc can be maintained while the torch is used for discontinuous operations, and the erosive effects on both the nozzle and the electrode are minimized.
In one embodiment, the present invention provides a method of operating a plasma arc torch selectively in a working mode and a standby mode. An electric arc is established between the electrode and a workpiece, for example, by initiating a pilot arc between the electrode and the nozzle with a current less than a subsequent working current, initiating the flow of plasma gas around the electrode and through the nozzle at a pilot flow rate less than a subsequent working flow rate, and then transferring the pilot arc from the nozzle to the workpiece. The torch is operated in the working mode at a relatively high arc current, such as at least about 250 amps, and the plasma gas is supplied at a relatively high flow rate, such as at least about 2 cubic feet per minute (CFM). When the working mode is to be terminated, instead of shutting off the torch, the torch is switched to the standby mode, in which the arc current is less than the working current, such as less than about 25 amps. The standby gas is supplied to the torch during the standby mode at a standby flow rate, which can be less than the working flow rate, such as less than about 1 CFM and preferably between about 0.25 and 0.60 CFM. As a result, the arc is switched from the workpiece to the nozzle. In making the switch from the working to the standby mode, the plasma gas can be switched from an oxidizing gas, such as oxygen, used during the working mode to a non-oxidizing gas, such as nitrogen or argon, used during the standby mode. The working mode can then be resumed without having to re-start the torch.
The present invention also provides a plasma arc torch configured for selective operation in a working mode and a standby mode. The torch includes a nozzle assembly defining a bore and an electrode electrically insulated from the nozzle assembly and directed toward the bore such that the electrode can be directed toward a workpiece. A working arc power source is in electrical communication with the electrode and the workpiece and configured to supply a working arc current therebetween. A standby arc power source is in electrical communication with the electrode and the nozzle assembly and configured to supply a standby arc current therebetween. The power sources are controlled by a power controller. First and second gas sources are fluidly connected to the bore, and a gas controller is configured to control the flow of gas from the gas sources. The power controller and the gas controller are configured to switch selectively between a working mode and a standby mode. The working mode is characterized by an arc extending between the electrode and the workpiece, the arc having a working current, and a plasma gas flowing through the nozzle at a working flow rate. The standby mode is characterized by the arc extending between the electrode and the nozzle, the arc having a standby current less than the working current, and the standby gas flowing through the nozzle at a standby flow rate that can be less than the working flow rate. The working and standby arc power sources can be configured to supply currents of at least about 250 amps and less than about 25 amps, respectively. The first and second gas sources can be configured to supply a non-oxidizing gas and an oxidizing gas, respectively, each controlled by the gas controller. Further, the gas controller can be configured to variably regulate the flow rates of the gases.